Variation is first divided into environmental and genetical effects

Variation in the beak size of finches is caused by genetic and environmental factors. it is not only controlled by the environment. In order to find out what causes beak size to be larger or smaller. we suppose that

1. only environmental conditions cause a change in beak size
2. . The genotype of all birds is the same.
3. But they differ in beak size only because of the difference in environmental conditions.

Beak size couldn’t evolve in that condition. Genetics also control the character and influence the evolutionary effect. The effect of both factors is given as

Mean of population + P = mean of population + G

Environmental conditions also play their role in the change of characteristics. These environmental effects are then measured in the same way as for the genotype, as a deviation from the population means. The phenotype can then be expressed as the sum of environmental (E) and genotypic influences

P = G + E

Reference: Mark Ridley book of evolution 5th edition